SMTP Error Codes

SMTP is responsible for delivering your email campaigns. If you send bulk emails, you have probably faced the problem of various SMTP errors that prevented your subscribers from receiving your emails. Mind that not every reply means an error. It can be a response to a command, including important information about a server. To understand what's wrong, you should know what the codes mean.

Error codes are three digits, each with their own meaning. The first digit shows whether your command was accepted and handled.

There are Five Values for the First Digit in an Error Code

1: The server accepted a command, but no actions have been taken. A confirmation message is required.

2: The task was completed successfully by the server without any mistakes.

3: The server understood the request, but further information is necessary for completing it.

4: A temporary failure occurred. If there are no changes while repeating the command, try again.

5: The server faces a critical error, and your command can't be handled.

If an error starts with 2, your command is successfully handled, with 4 — there is a temporary failure, with 5 — there is a permanent failure. Your emails will not be sent unless you eliminate the failures.

There are Four Values for the Second Digit in the Code

1: There is a syntax error.

2: Information reply (a help request, for example).

3: Shows the connection status.

4: Shows the status of a mail server or a mail system.

The last digit means more detailed information on the delivery status.

Most Important SMTP Error Codes

214 — Reply to "Help." Information on how to work with a server, description of specific commands

220 — Service is running/ready.

221 — The service is closing the transmission channel.

250 — Requested mail action is completed and OK. 

251 — The recipient is not local, but the server will forward a message.

252 — The server can't verify the user, but it will try to deliver the email.

354 — Start mail input end with <CRLF>.<CRLF>. The server received the information about the sender and the recipient, and asks for "Message Body."

421 — The service is not available, the transmission channel is closed.

422 — The size of the email exceeds the limits of the recipient's inbox.

442 — The mail server started to deliver the email, but the connection was dropped during transmission.

450 — The requested action was not taken; the mailbox is unavailable. It may be temporarily busy.

451 — The requested operation was aborted, a local error is in processing. 

452 — The requested action was not taken. Insufficient storage.

500 — A syntax error occurred because the server could not recognize a command.

501 — A syntax error occurred in arguments or parameters. It could be an invalid email address.

502 — The command is not implemented. It is valid but has not been activated by your provider.

503 — Bad sequence of commands, the mail server requires authentication.

504 — A command parameter is not implemented.

510 — Bad email address. Your recipients' emails may contain typos.

512 — Unknown host, domain can't be found.

513 — Address type is incorrect. There may be a misspelling, or the server requires authentication.

515 — Invalid mailbox address.

521 — Mailbox does not accept emails.

523 — Email's size exceeds the recipient's limits.

530 — Authentication is required, the recipient's server has blacklisted your server, or the email address is invalid.

531 — The mail system is full.

534 — The authentication mechanism is too weak, or a message is too big.

535 — Authentication errors.

541 — The recipient's address rejected your message. It can be caused by spam-filter.

550 — The requested actions were not taken because the mailbox is unavailable. The command may have been rejected for policy reasons, or the inbox was not found, or there was no access. 

551 — User is not local, or address is invalid.

552 — The action was aborted because it exceeded storage allocation.

553 — The requested action was not taken; the mailbox name is invalid. 

554 — Transaction failed. This happens when the recipient's server considers your email to be spam or your IP address to be blacklisted.

555 — The server did not recognize the email address format.

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